During the processing of semiconductor wafers used in manufacturing integrated circuits and the like, it is typically necessary to remove chemicals or residues from the wafer surface. For example, it is sometimes necessary to etch openings or other geometries into a thin film deposited onto (or grown on) the surface of a wafer substrate. (The wafer substrate typically comprises silicon, gallium arsenide, glass, an insulating material such as sapphire, or any other substrate material upon which an integrated circuit wafer may be fabricated.) Present methodology for etching such a thin film requires that the film be exposed to a chemical etching agent to remove desired portions of the film or films. The composition of the etching agent used to remove the portion of the film depends upon the nature of the thin film.
In order to ensure that only desired portions of the thin film are removed, a photolithography process is use by which a pattern is transferred to the surface of the thin film. The pattern serves to identify the areas of the thin film that are to be selectively removed. The pattern is typically formed with a photoresist material, typically a light-sensitive material that is spun onto the in-process integrated-circuit wafer also in the form of a thin film. The thin film of photoresist is then exposed to a high intensity light source that is projected through a photomask. The photomask defines a desired pattern. As the light source is projected through the photomask, the desired pattern is defined on the photoresist thin film. The exposed or unexposed photoresist, depending upon the polarity of the photoresist material, is dissolved (i.e., is removed or stripped) with developers, leaving a pattern that allows etching to take place in the selected areas only.
Some of the current methods for stripping the photoresist (or other materials, such as dry-etch residues or polymers) include a hot chemical removal with a chemical etching agent. Sulfuric acid and hydrogen peroxide or dry reactive removal, known as photoresist ashing are typical removal methods. The hot chemical removal methods are undesirable in that they involve great expense due to the relatively large amount of chemical etching agent needed and require expensive disposal methods due to the caustic nature of the chemical etching agents. The ashing method is undesirable in that it involves a high-energy gas and often incurs damage to the wafer substrate or the layers of thin films formed on the wafer substrate to make the wafer integrated circuits.
Some chemicals in the gaseous phase may react with photoresist material or other such materials to facilitate removal of the materials from the wafer surface. Many of such gases, however, do not have effective transport means to the wafer surface to effect the necessary reaction in a reasonable period of time. Also, many such gases are too unstable to be introduced to the wafer in an atmosphere filled with such gas and effect the necessary reaction with the photoresist material. Such gases typically have short half-lives and change in structure in such environments so quickly that the gas is unable to react with a thin film material on the surface of a wafer. Many such gases (both the unstable gases and those lacking sufficient transport characteristics), however, are sufficiently soluble in a variety of liquid solvents.
For example, there has been a current interest in the use of ozone (i.e., O3) as a photoresist etching agent for the stripping of photoresist, dry etch residues/polymers, and the like from a wafer surface. Ozone reacts with photoresist material on the wafer surface to oxidize the photoresist (forming CO2). Ozone, however, is an unstable gas and will decompose before reacting with the wafer surface photoresist material if simply introduced in its gaseous state. Accordingly, a solvent is used to dissolve the ozone and transport the ozone to the wafer surface such that the ozone may react with the photoresist material and strip the photoresist material from the wafer surface.
Water may act as a solvent to dissolve ozone. One method for use of ozone as a stripping agent involves immersing the in-process wafer into a water bath through which ozone is bubbled. It is difficult, however, to get a sufficient amount of ozone dissolved in the water to affect the desired oxidation reactions. Further, the amount of ozone transported to the wafer surface is limited due to the large amount of water filling the bath. Consequently, the stripping process is very slow.
Without being tied to any particular theory, it is believed that a main barrier to dissolution of the gas into water is kinetic in nature. Another method calls for chilling a water bath and using a diffusion plate in the water to bubble ozone gas therethough. The diffusion plate creates numerous tiny bubbles that rise through the water. The wafers are then immersed in the water bath. During this residence time, the gas dissolves in the liquid by crossing the gas/liquid interface so that the ozone in the water strips the photoresist (or other material) on the wafer. Other methods for dissolving the gas into the liquid (e.g., water) include the use of static mixing devices and membrane contactors.
This method, however, relies heavily on the configuration and performance of the diffusion devices (e.g., the diffusion plate in the water bath) and requires long time periods of exposure of the gas to the water. The increased time and the need for diffusion apparatus add undesirable time and complexity to the process. Further, the photoresist stripping effectiveness of such processes is limited, as discussed above, as only a small amount of ozone moves to (i.e., has physical contact with) the surface of the wafer while the wafer is immersed in the water bath.
Accordingly, methods and apparatus are needed to fabricate and/or clean wafers without incurring the expense and apparatus complexity encountered with the prior art methods and apparatus. Additionally, methods and apparatus are needed that provide effective stripping of photoresist, dry etch residues/polymers, or the like in a reasonably short time period. Further, methods and apparatus are needed that can overcome the kinetic limitation to dissolution of a gas in a liquid without the need for long exposure times of the gas to the liquid.
To overcome the disadvantages of the prior art, methods and apparatus are disclosed herein. The methods and apparatus provided eliminate the need for large amounts of caustic chemical cleaning agents to remove photoresist, dry etch residues/polymers, or the like. The methods and apparatus provided also require only a relatively small amount of gas and liquid to strip the photoresist, dry-etch residues/polymers, or the like. Additionally, the methods and apparatus overcome the kinetic limitation of dissolution of the gas in the liquid without requiring long exposure times of the gas to the liquid. Further, the methods and apparatus provide a liquid solvent that effectively transports the reactant gases that most effectively and quickly strip photoresist material (or other material) from a wafer surface. The liquid solvent, however, does not react with materials on the wafer surface, but merely acts as a transport medium to put the reactant gas in physical contact with the wafer surface.
More specifically, an in-process wafer is placed in a chamber, preferably a chamber of low volume. A liquid solvent (e.g., water) incorporates (e.g., dissolves) a reactant gas (e.g., ozone) to create a “reactant mixture.” The reactant gas in the mixture will react with and remove photoresist material (or other material) on the wafer surface. In one representative method, the reactant mixture enters the chamber and forms a thin film on one or more surfaces of the wafer. The chamber preferably includes a reactant gas atmosphere during and/or after formation of the thin film. The solvent acts as a transport medium to place the reactant gas in direct physical contact with the wafer surface. The reactant gas is then able to react with the photoresist material (dry etch residue/polymer or the like) on the in-process wafer surface to effect removal of the material. The solvent does not react with the photoresist material (or other material at issue) to be removed nor with the reactant gas. The solvent acts merely to transport a sufficient amount of the reactant gas to the wafer surface such that the gas reacts with the photoresist material to effect removal.
For example, ozone (i.e., a reactant gas for conventional photoresist material) dissolves in water (i.e., an ozone solvent or “transport medium”) to form a reactant mixture. The reactant mixture condenses to form a thin layer on one or more wafer surfaces. The ozone reacts with the photoresist to form CO2. The water does not react with the photoresist (or the ozone), but merely transports a sufficient amount of the ozone gas to the wafer surface so that the ozone gas reacts with the photoresist to effect removal in a relatively short period of time. Following reaction with the photoresist, the layer of reactant mixture is removed from the wafer surface by flash heating, rinsing, drained, or other suitable removal method.
In the representative methods and apparatus, the reactant mixture condenses or otherwise collects on the wafer surface to form a thin film thereon. The high surface area to volume ratio of the thin film reactant mixture results in transport of a relatively high volume or concentration of reactant gas directly onto the wafer surface. Accordingly, the removal of photoresist (or the like) from the wafer surface occurs relatively rapidly. The methods and apparatus allow the removal process to be carried out in a low volume chamber requiring a minimal amount of reactant gas and solvent. Additionally, both the reactant gas and the solvent may be purified and re-circulated during the wafer fabrication process, thereby wasting little chemical and having little chemical waste for which disposal is necessary.
The foregoing features and advantages of the methods and apparatus will become more apparent from the following detailed description of representative methods and apparatus that proceed with reference to the accompanying drawings. The present invention is directed toward novel and non-obvious features and advantages of the disclosed methods/apparatus for fabricating and cleaning in-process wafers, both individually and collectively, as set forth above and additionally as set forth in the drawings and description following.